1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a lamp and a driving device for a backlight assembly having the same.
2. Description of the Related Art
Examples of flat panel displays include plasma display panels (PDPs), field emission displays (FEDs), and liquid crystal displays (LCDs). The flat panel displays are broadly classified into light-emitting type displays and light-receiving type displays. PDPs and FEDs are light-emitting type displays, and LCDs are light-receiving type displays. The LCD cannot display an image without an external light source because it is not self-luminous. Therefore, the LCD requires a backlight assembly for emitting light.
General requirements of the backlight assembly include high brightness, high efficiency, uniform brightness, long lifetime, thin profile, light weight and low cost. Generally, a notebook computer is equipped with a high-efficiency and long-lifetime backlight assembly so as to reduce its power consumption, and a PC monitor or a TV can also be equipped with a high-brightness backlight assembly.
A backlight assembly is equipped with a lamp or a plurality of lamps as a light source. Backlight assemblies are classified into either an edge type or a direct type. In the edge type backlight assembly, a lamp is disposed at an edge of a liquid crystal panel and a light guide plate guides light emitted from the lamp toward the liquid crystal panel. The lamp can be disposed at one edge or a plurality of lamps can be disposed at different edges of the liquid crystal panel, for example, both left and right edges. Also, the plurality of lamps can be disposed at all edges of the liquid crystal panel. Meanwhile, in the direct type backlight assembly, a plurality of lamps are disposed at the rear of a liquid crystal panel and spaced apart from one another by a predetermined distance, such that they directly illuminate the liquid crystal panel. In both types of backlight assemblies, a cold cathode fluorescent lamp (CCFL) is widely used because of its high brightness.
FIG. 1A is a view of a straight-shaped CCFL in the related art backlight assembly, and FIG. 1B is a view of a U-shaped CCFL in the related art backlight assembly.
Referring to FIG. 1A, the straight-shaped CCFL includes a cylindrical glass tube 1 and electrodes 2 and 3 at both ends thereof. The glass tube 1 is elongated along a straight line, and the electrodes 2 and 3 are disposed at both ends of the glass tube 1. A predetermined voltage is supplied across the electrodes 2 and 3 of the glass tube 1. One end of each of the electrodes 2 and 3 is inside the glass tube 1. Therefore, the predetermined voltage is directly supplied to the inner space of the glass tube 1, causing a discharge therein. This straight-shaped CCFL is widely used in the backlight assembly because it has a high brightness of several ten thousands cd/m2.
To meet the lighting requirements of large-sized liquid crystal panels, the direct type backlight assembly is widely used. However, the direct type backlight assembly requires many lamps for directly illuminating the large-sized liquid crystal panel. Since the lamps are separately driven, a lamp drive circuit of the direct type backlight assembly is complex and bulky. To solve these problems, various attempts have been made to alter the structure of the lamp. For example, a U-shaped CCFL and a zigzag CCFL have been proposed.
Referring to FIG. 1B, the U-shaped CCFL includes cylindrical glass tube portions 5 and 6 and electrodes 8 and 9. The glass tube portions 5 and 6 are paired in one body. One end of each of the glass tube portions 5 and 6 is bent and connected at a bent portion 7. The electrodes 8 and 9 are exposed inside the other end of each of the glass tube portions 5 and 6. Consequently, the glass tube portions 5 and 6, and the bent portion 7 are formed in a U-shape.
Accordingly, one U-shaped CCFL corresponds to two straight CCFLs. Therefore, the required number of the U-shaped CCLFs is ½ of that of the straight CCFLs. Since only one driving voltage is required for one U-shaped CCFL corresponding to two straight CCFLs requiring two driving voltages, a lamp driving circuit can be simplified. Consequently, a required cost can be reduced. Typically, the U-shaped CCFLs is driven in a floating type manner.
FIG. 2 is a schematic diagram illustrating a driving device for a backlight assembly having the U-shaped lamp shown in FIG. 1B. Referring to FIG. 2, the backlight assembly driving device includes a controller 11 for outputting a PWM (pulse width modulation) control signal, a power transistor 13 for converting an external DC voltage into a DC square wave voltage in response to the control signal, a resonant inverter 15 for converting the DC square wave voltage into an AC sine wave voltage, and a U-shaped lamp 17 for emitting light by the AC sine wave voltage.
Although only one resonant inverter is illustrated in FIG. 2, two resonant inverters are required for providing an AC voltage to each of electrodes 8 and 9 in the U-shaped lamp 17.
A first Ac voltage and a second AC voltage are applied respectively to the electrodes 8 and 9. Here, a phase of the first AC voltage is opposite to that of the second AC voltage. Therefore, an attenuated voltage (ideally, OV) exists at the bent portion 7 of the lamp 17.
Glass tube portions 5 and 6 have the same length and a phase of the first AC voltage is always opposite to that of the second AC voltage between the electrode and at the bent portion 7. Therefore, the first AC voltage is cancelled out by the second AC voltage at the bent portion 7. This is called a floating type driving. Accordingly, when the first and second AC voltages having opposite phases are supplied respectively to the electrodes 8 and 9, the glass tube portions 5 and 6 can emit light of the same brightness.
The resonant inverter 15 has an impedance due to an inductor and a capacitor. Also, the U-shaped lamp 17 has an inherent impedance. Each impedance of the resonant inverter 15 and the U-shaped lamp 17 is varied by external factors (such as noise). Accordingly, the glass tube portions 5 and 6 have different impedance values. This impedance difference causes a canceling out of the first and second AC voltages at a portion of the glass tube 5 or 6 other than at the bent portion 7. Light is not generated at the portion where the first and second Ac voltages are cancel each other out. Also, a tube current flows through the glass tube portions 5 and 6 when a discharge is generated in the glass tube portions 5 and 6 by the first and second AC voltages. This tube current varies according to an impedance. Therefore, due to the impedance difference, respective tube currents flowing through the glass tube portions 5 and 6 become different to each other. The glass tube portion with a larger tube current has high brightness and the glass tube portion with a smaller tube current has low brightness. This causes a non-uniformity in brightness.
A unit (not shown) for detecting electrical characteristics (e.g. voltage, current, and impedance) of the U-shaped lamp 17 is connected between the resonant inverter 15 and the U-shaped lamp 17. Electrical characteristics detected by the unit are supplied to the controller 11 and a corresponding control operation is accordingly performed. Since the unit is connected between the resonant inverter 15 and the U-shaped lamp 17, the accurate impedance of the U-shaped lamp 17 cannot be detected. The first and second AC voltages can only be controlled when the impedance difference between the glass tube portions 5 and 6 is accurately detected. However, since the unit is provided in front of the U-shaped lamp 17, an impedance difference between the glass tube portions 5 and 6 cannot be accurately detected.
A long U-shaped lamp 17 is required for a large-sized liquid crystal panel. When the U-shaped lamp 17 is long, the first and second AC voltages drop due to the internal impedance of the glass tube portions 5 and 6. A large voltage drop occurs at the bent portion 7. While the end portions of the glass tube portions have high brightness, the bent portion 7 has low brightness. This causes a non-uniform brightness.